Pakistan says a deadly assault on a university in a northwestern city on Wednesday was “being controlled from a location in Afghanistan through an Afghan cell phone” by a fugitive member of the Pakistani Taliban.

Army spokesman Asim Bajwa released details of Pakistan's investigation into the attack Thursday. He said Pakistan’s military chief, General Raheel Sharif telephoned the Afghan leadership and the commander of NATO’s Afghan mission and “asked for their cooperation in locating and targeting those responsible for this heinous act and bring them to justice.”

Pakistan alleges that leaders and commanders of the anti-state Taliban militants have taken refuge on the Afghan side of the border after fleeing army-led counterinsurgency operations. Officials have previously also blamed these fugitives for planning cross-border attacks in Pakistan.

But Afghan Interior Ministry spokesman Sediq Seddiqi on Thursday dismissed allegations that there are terrorist bases in Afghanistan being used against Pakistan. He strongly condemned the Bacha Khan University attack in Charsadda and reiterated Kabul’s allegations that terrorist sanctuaries are located in Pakistan and are causing instability in the region.

Meanwhile, Pakistan observed a day of mourning Thursday following the gun and bomb attack on the university in which 21 people were killed and dozens more wounded.

Prime Minister Nawaz Sharif declared the mourning period to show solidarity with those who died when a group of four terrorists stormed the Bacha Khan University in Charsadda.

Military spokesman Asim Bajwa said that troops, including army commandos gunned down the four attackers and conducted a "block by block" clearing operation before securing the entire campus.

Taliban denies involvement

The spokesman for the Pakistani Taliban, Mohammad Khorasani, said Wednesday the group had nothing to do with the university attack, adding that non-military institutions are not on its list of targets. In a statement sent to VOA, Khorasani declared Wednesday's attack against "Islamic Sharia."

Earlier, a local Taliban commander had said he sent the four attackers to the university. There was no explanation for the conflicting claims.